How to think about… Relativity

By Richard Webb

Space and time used to be so simple. You trundled around reasonably freely in the three dimensions of the one, and experienced occasional heartache at the remorseless forward march of the other. C’est la vie.

Or is it? Einstein revolutionised our perceptions a century ago when, in his theories of relativity, he first forbade anything in the cosmos from travelling faster than the speed of light, and then bundled both space and time into one unified space-time that can be warped by gravity (see “How to think about… Space-time“). The contortions introduced by Einstein’s special and general theories make intervals in both space and time dependent on where we measure them from. Two observers with flashlights in fast-moving trains might both measure the other to have flashed their flashlight first – and both be right from their own point of view.

The recent blockbuster Interstellar is based on premises that Einstein made technically plausible, if not (yet) technologically feasible&colon; that by travelling close to the speed of light, or moving in an intense gravitational field such as that of a black hole, we age more slowly than those we leave behind on Earth (see diagram). We don’t need to travel that far to see less dramatic effects of relativity in action. Astronauts on the International Space Station age a little less because of the velocity at which they travel, and a little more for enjoying less of the gravity of mothership Earth. The effects don’t quite cancel out. Velocity wins, leaving each ISS astronaut who completes a six-month tour of duty 0.007 seconds younger than someone who stayed on

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